If you join a community server, your friends can join you and leave the in-game party to allow more friends to join; or they can join you using your gaming platform's mechanisms. Once you are all in the server together, you will stay together round after round, regardless of party status, and quantity (within reason -- as long as it is few enough to fit on a team, lol), and you can use the voice chat party system of your choice applicable to whatever gaming platform you use.
Matchmake servers are like COD servers. The matchmaking system is full of compromises, and they aren't optimal for playing in a group. They are best suited for solo quick-and-dirty games where you don't care if you join game after game halfway through or how many rounds you play on the same attack/defender side or map repeats. As you see, it doesn't support groups well.
Community servers are like BF3 and BF4 servers. They are designed for groups. It's in the name. Community. You can host versions tailored to a group's preference. There are official servers, verified-mode servers, and custom servers. They operate on a fixed map rotation, switching sides consistently, and keep the server population together round after round. You join a server of your choice with the name, ping, and description of your choice, and you stay on that server indefinitely.